I. Field of the Invention
The present invention generally relates to wireless communications. More specifically, the present invention relates to a method of and an apparatus for providing continuous connectivity as a handset moves from one base station to another.
II. Description of Related Art
Code Division Multiple Access (CDMA) modulation is but one of several techniques for facilitating communications in which a large number of system user are present. Although other techniques, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), the Global System for Mobile Communications (GSM), AM modulation schemes, such as Amplitude Companded Single Side Band (ACSSB), Orthogonal Frequency Division Multiplexing (OFDM) and Integrated Dispatch Enhanced Network (iDEN) are also available, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,” assigned to the present assignee, which is incorporated by reference herein. In U.S. Pat. No. 4,901,307, a multiple access technique is described in which a large number of mobile telephone system users, each having a transceiver, communicates through satellite repeaters or terrestrial base stations (also known as cell base stations or cell sites) using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times, thus preventing an increase in system user capacity. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
In conventional cellular telephone systems, the available frequency band is divided into channels, typically 30 KHz in bandwidth, while analog FM modulation techniques are used. The system service area is divided geographically into cells of varying size. The available frequency channels are divided into sets with each set usually containing an equal number of channels. The frequency sets are assigned to cells in such a way as to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the cells are equal size hexagons. A frequency set used in one cell will not be used in the six nearest or surrounding neighbors of that cell. Furthermore, the frequency set in one cell will not be used in the twelve next nearest neighbors on that cell.
A more difficult situation is presented by movement of the mobile station into a cell serviced by a base station from another cellular system. A complicating factor in such intersystem handoffs is that the neighboring cellular system often has dissimilar characteristics and requirements. For example, adjacent cellular systems may often operate at different frequencies, and maintain different levels of base station output power, pilot strength, or capacity. Further, adjacent cellular systems may require different messaging structures, even for similar types of messages or the functionalities thereof. For example, the so-called the GSM standard does not have a mechanism for soft handoff. There is, therefore, a problem in handing off a call using the error interface from a CDMA network to a GSM network, or vice-versa.
One way of addressing this problem is to modify GSM to enable it to effect handoff to a non-GSM system, that is, a CDMA system. Another way of dealing with this problem is to modify CDMA to handle mechanisms that are standard in a GSM system. However, both CDMA and GSM are well-established systems, and operators and equipment providers are reluctant to make expensive modifications to existing equipment in order to accommodate a neighboring incompatible system. If new messages are added to the error interface in support of dual mode mobile stations, then modifications must be made to support these new messages using existing hardware. Plainly, this is undesirable from the perspective of the operator and the equipment provider.